Waveguide-based spectrometers capable of separating light signals at different wavelengths have already been established as one of the key technologies in wavelength division multiplexed telecommunications networks (wavelength multiplexers and demultiplexers, add-drop filters, channel monitors, etc.). High-resolution spectroscopic devices are also required for the development of new detection platforms for genomics and health related applications, and environmental monitoring, including space-born sensing.
In grating spectrometers, high resolution is achieved by reducing the input slit width, or the input waveguide width in planar waveguide based devices. However, a reduction in the input aperture invariably results in a reduced light throughput (étendue) because of a degraded light coupling efficiency between the delivery system (typically an optical fiber) and the spectrometer. The problem is even more obvious when using optical fibers with large core area (multimode fibers). Such fibers are typically used for efficient harvesting of light at the input end of the delivery fiber.
In addition to these étendue concerns, it is known that the input waveguide width cannot be reduced below the point at which the confinement of the mode is compromised and mode delocalization occurs. This sets an ultimate limit to a grating-based waveguide spectrometer resolution. One solution to overcome this fundamental resolution limit is to abandon altogether the slit imaging microspectrometer concept.
Diffraction gratings or arrayed waveguide gratings are commonly used to disperse the spectrum of optical radiation into different wavelength components in the focal plane of a spectroscopic instrument. Several grating-based microspectrometers have been demonstrated as shown by the following references: S. H. Kong et al., IEEE Instrument. & Measurement Mag. Vol. 4, pp. 34-38 (2001); P. Krippner et al., SPIE Vol. 2783, pp. 277-282 (1996); J. H. Correia et al., IEEE Trans. on Electron. Devices, Vol. 47, pp. 553-559 (2000); R. V. Kruzelecky et al., SPIE Proc. Vol. 4205, pp. 25-34 (2001); S. H. Kong et al., Sensors and Actuators, Vol. A92, pp. 88-95 (2001); Z. M. Qi et al., Optics Letters Vol. 27, pp. 2001-2003 (2002); D. Sander et al., Sensors and Actuators, Vol. A88, pp. 1-9 (2001); J. M. Harlander, Appl. Optics Vol. 41, No. 7, 1343-1352 (2002); and P. Cheben et al., Opt. Lett., Vol. 30, No. 14, 1824-1826.
However, virtually all of these spectrometers suffer from a limited resolution (typically >1 nm) which is not suitable for high resolution spectroscopies. Even to achieve a moderate performance, including resolution and light gathering capability (étendue), serious fabrication difficulties have to be resolved. For example, microspectrometers fabricated by LIGA process (deep X-ray lithography and micro-electro plating require exceptional high structuring resolution of <0.25 μm across the entire waveguide thickness of about 100 μm. Waveguide inhomogeneities during embossing induce a significant stray light level compromising resolution and crosstalk, and reducing detection accuracy particularly in spectral ranges of low sensitivity. In another reported example of an on-chip microspectrometer, bulk micromachining of silicon is used to create an array of Fabry-Pérot interferometers, yielding a limited number of available spectral bands (<20) and modest resolution (˜2 nm). The latter is due to difficulties with fabrication of micro-mirror surfaces with high surface quality, parallelism and reflectivity.
Commercial portable micro-spectrometer systems exist, such as the Ocean Optics S1000 and S2000 series, the microParts LIGA serie, the Carl Zeiss MMS1, IOSPEC by MPB Technologies, and the Elargen micro-lightguide spectrometer, but their use is again limited to spectroscopic application with low resolution. Increasing the resolution in these devices is inevitably accompanied by degrading light throughput rendering these devices useless for high-sensitivity high-resolution applications.
It is known that Fourier transform (FT) spectrometers outperform by several orders of magnitude the light gathering capability (étendue) of grating spectrometers at a comparable spectral resolution. Unfortunately, typical FT spectrometers require moving parts, which is very difficult to realize in an integrated optics version, although recently, a new FT spatial heterodyne spectrometer with no need for moving part has been demonstrated both in bulk optics and planar waveguide form.
Miniature spectrometers are typically bulk optic devices composed of lenses, mirrors and other large optical components. As such they are large and require careful assembly for each device. The actual physical layout of the device is somewhat complex and involves making compromises in resolving power, etendue and size to get a working device. In echelle waveguide grating devices, some complex fabrication steps are required to create waveguide vertical trenches with smooth vertical sidewalls. In an arrayed waveguide grating device, the waveguide lengths need to be fabricated within very tight tolerances. In these devices, the etendue is limited and single input aperture is used.